The field of the invention is motor drives for positioning materials at a work station, and particularly, the movement and positioning of metal during high speed manufacturing processes.
Many articles are formed from sheets or tubes of metal which are cut and shaped in high speed automated manufacturing processes. For example, sheet steel may be removed from a roll, cut into pieces, and formed into automobile fenders, refrigerator doors, or the like, at a rate of fifty or sixty per minute. The movement of the metal fabricating material is accomplished using rollers or belts which are driven by conventional a.c. or d.c. rotary electric motors. The rotary motors drive the rollers or belts through a mechanical drive train which may include shafts and gears of various sizes and shapes. The rotary motors are controlled by drive circuits which determine the direction and speed of rotation. Feedback devices such as pulse generators may also be employed with such drive circuits to form closed loop position controls that enable the system to rapidly move the fabricating material and precisely position it in a work station.
There are a number of difficulties with conventional motor drive systems for high speed metal fabricating processes. First, the rollers or belts employed to propel the fabricating material can mar the surface of the material. This is a major problem when soft metals such as aluminum are used, or when the fabricating material is subjected to high acceleration and deceleration forces. To minimize such problems, the surface area of the driving elements is often increased to distribute the driving force over the surface of the facricating material. This requires larger, or additional rollers, which in turn increases the size of the drive train from the rotary motors. The mass of the drive train and rollers may exceed that of the driven fabricating material. As a result, the drive motor and its control circuits must be substantially increased in size to meet the desired acceleration and deceleration specifications.
Linear motors have long been used in industry to propel fabricating materials along conveyors. Sheet metal has been driven along a conveyor by linear motor windings located beneath the conveyor and along its length. As disclosed in U.S. Pat. No. 3,610,695 for example, photo detectors are placed along the length of the linear motor conveyor and as the sheet metal passes, additional segments of the linear motor are energized to propel the material onward. When the sheet material reaches a work station it is propelled against mechanical stops which position it as described in U.S. Pat. No. 3,662,625. A similar structure in which the linear motor stator windings encircle the tubular material is disclosed in U.S. Pat. No. 3,616,978.
The use of linear motors in prior processes has been limited to the simple function of providing a propelling force to the fabricating material. Such a propelling force may position the fabricating material only in the sense that it propels the material against mechanical positioning mechanisms, or allows the material to come to a halt after passing a selected position on a conveyor.